There a several laser applications that require relatively high average power, for example, greater than one-hundred milliwatts (mW) average power, of continuous wave (CW) UV laser radiation at some UV wavelength between about 200 nanometers (nm) and 400 nm. Such applications include wafer and mask inspection in the semiconductor industry.
Commercially available lasers suitable for this purpose include solid-state and optically pumped semiconductor (OPS) lasers in which UV radiation is generated by frequency conversion of fundamental wavelengths in the near infrared spectral region to third-harmonic or fourth-harmonic wavelengths. By way of example, a 1064 nanometer (nm) fundamental wavelength of neodymium-doped yttrium vanadate (Nd:YVO4) can be converted to a third-harmonic wavelength of about 353 nm or a fourth-harmonic wavelength of about 266 nm using two stages of frequency conversion. Intra-cavity (intra-resonator) frequency conversion is usually employed for generating CW harmonic radiation. The need for two intra-cavity (IC) frequency conversion stages adds cost and complexity to a laser resonator. IC-frequency tripling and quadrupling, however, are rather complex and require complex control technology to ensure that the laser output power and beam-pointing are stable. Further, the efficiency of a two-stage IC CW frequency converted laser is typically relatively low, for example less than about 3% of pump-power.
One approach to avoiding the measures needed to stably operate an intra-cavity frequency-tripled or frequency-quadrupled laser would be to configure an intra-cavity frequency doubled laser having a gain-medium such as praseodymium-doped yttrium lithium fluoride (Pr:YLF) that can deliver several fundamental wavelengths between about 500 nm and 750 nm. Within this wavelength range, Pr:YLF has transitions (gain-lines) at about 522 nm, about 644 nm, and about 720 nm among others. Fundamental wavelengths of 522 nm and 720 nm, when frequency doubled, would provide UV wavelengths of 261 nm and 360 nm respectively. Optical pump radiation for energizing these transitions of Pr:YLF would need to have a (blue) wavelength of between about 430 nm and 490 nm. In order to generate more than 100 mW of UV output a pump-power of between about 2 and 4 Watts (W) would be required
Earlier, this approach has not been practical due to due to lack of blue-light emitting diode-lasers having sufficient output power. Now, however, diode-lasers having such an output power in the required wavelength range are commercially available. This offers the prospect of a relatively efficient and relatively inexpensive solid-state UV laser. However, absent any counter measure, such a laser would still have a problem common to all IC frequency-doubled solid-state lasers, i.e., that of noisy and chaotic operation.
This noisy and chaotic operation occurs because solid-sate gain-media doped with rare earth or transition metals such as neodymium (Nd), thulium (Tm), holmium (Ho), erbium (Er), ytterbium (Yb), chromium (Cr), and praseodymium (Pr) all have long excited-state lifetimes ranging from several microseconds (μs) to a few milliseconds (ms). Theses long lifetimes lead to longitudinal mode-coupling which can cause fluctuations of circulating power and harmonic output power.
The problem was first recognized in a paper “Large-amplitude Fluctuations Due to Longitudinal Mode Coupling in Diode-Pumped Intracavity-Doubled Nd: YAG Lasers” T. Baer, J. Opt. Soc. Am., 3, 9, (1175-1179), September 1986. The authors concluded that when doing intra-cavity frequency-conversion in lasers with such gain-media, the long excited-state lifetimes gave rise to chaotic noise fluctuations and instability in the frequency converted output because of mode-coupling effects. These chaotic fluctuations became known to practitioners of the art as “the green problem” having been described in terms of frequency-doubling 1064-nm (Near-IR) radiation to provide 533-nm (green) radiation.
One solution to the green-problem that has enjoyed commercial success is to perform intra-cavity frequency-doubling in a traveling-wave ring-resonator operating in a single longitudinal mode to avoid the mode-coupling. Such a ring resonator is readily operable in a single longitudinal mode because the traveling wave eliminates the problem of spatial hole-burning which complicates single-mode operation in standing wave resonators. A traveling-wave ring-resonator, however, would not be practical for 522 nm radiation generated by Pr:YLF because optical diodes needed to achieve unidirectional circulation in the resonator have too much absorption at this wavelength. Accordingly, a standing-wave resonator would be required for a single-mode IC frequency-doubled Pr:YLF laser delivering 261 nm radiation.
The spatial hole-burning complication of single-mode operation of a standing-wave solid-state laser arises because the desired single mode saturates gain at antinode positions of the standing-wave in the solid-state gain-medium, leaving gain between the antinodes higher than at the antinodes. This provides that another possible mode, with antinodes between those of the desired mode, will preferentially oscillate then start the spatial hole-burning afresh. This leads to noisy multimode operation.
Measures that have been taken to mitigate the spatial hole-burning problem include using a relatively short gain-medium at one end of a resonator where the possible oscillation modes have antinodes relatively close together, and providing an intra-cavity spectrally selective device that can suppress modes adjacent a desired mode. A preferred such device is a free-standing uncoated etalon. Such an etalon is usually made from fused silica and can have essentially 100% transmission (the term “essentially” here recognizes that there may by some fractional percentage loss due to scatter).
Depending on resonator length, a fused silica etalon having a thickness of about 1.0 millimeter will have a transmission bandwidth (FWHM) comparable with the spectral width of saturated gain in Pr:YLF. Such an etalon however will have a free-spectral-range of only about 90 picometers (pm). This puts adjacent transmission peaks at wavelengths outside the depletion (saturation) region but within the gain-bandwidth of the Pr:YLF, which wavelengths could oscillate. The gain-bandwidth of Pr:YLF is about 1.2 nm (1200 pm) FWHM. In order to suppress transmission at these adjacent peaks it is necessary to provide a second etalon having a significantly higher free-spectral-range to suppress transmission peaks of the first etalon. Such an etalon would have a thickness of only about 100 micrometers (μm). Unless a transmission-peak wavelength of the thin etalon is precisely aligned with a transmission-peak wavelength of the thick etalon at the desired operating wavelength of the resonator, the net transmission of the two etalons will be less than 100% which could add significantly to resonator losses. There is a need to provide spectral selectivity in a Pr:YLF resonator sufficient to ensure single-mode operation without using an etalon pair including a thick etalon and a thin etalon.